The morning news. [volume] (Savannah, Ga.) 1887-1900, 1887-08-11
Tornadoes and Cyclones.
From !Scrjbner's Magazine.

When the conditions of atmospheric in
stability have given birth to tornado, the
fact is announced to the observer by a sud
den gathering of dark, swift, whirling
clouds from which depend writhing SER
pent-like body formed of condensed vapor.
This writhing column extends rapidly
downward until it touches the earth.
When 1t attains the surface it becomes au-
dible from the violent rending actions
which it creates upon that surface. AS
soon as the whirl is created it begins to
move away-generally toward the north
east--for the evident reason that the upper
cold layer of air against which it originates
has, in the northern hemisphere, move
ment in that direction. In its path over
the surface the crcling movement of the
writing air and the sucking action of the
partial vacuum in the central portion of the
shaft combine to bring about an extreme
devastation On the outside of the whirl Of
air, which ruShes in circling path toward
the vortex, overturns all movable objects,
and in the centre of these objects, if they
are not too heavy. are sucked up as by
great air pump Thus the roofs of houses,
bodies of men and animals may be lifted to
great elevations until they are tossed by the
tumultuous movements beyond the limits of
the ascending currents and fall back upon
the earth Where the centre of q whirl-
wind passes over building the
sudden decrease i'll the pressure of
the outer air, often causes the at
mOsphere which is contained within the
walls suddenly to press against the sides of
the structure, so that these sides are quickly
driven outward as il by charge of gun
powder. 1t is not unlikely that the diminu-
ton of the pressure brought about by the
passage of the interior of the whirl over
building may be about as much as is indi
Gated by the fall of 4 inches in the barOme-
ter. This is equivalent to a change in the
pressure amounting to about 300 pounds to
the square foot. This force operates to
burst out the walls of a building. 1t is not
improbable that the diminu5iOn of pressure
may be much greater than this, but even
the amount named is sufficient to account
for the bursting out of the frail-walled
structures which these devastating move
ments encounter in the western parts of the
United States
The way in which these tornado whir]s
| are formed differs in certain essential par
ticulars from the way in which whirlwinds
are created, as has been well shown by
| Prof Ferret The most important points
| of difference are as follows: The dust whiris
are due to the heating of a thin layer of air
next the ground The small mass of this
| layer prevents it's upward whiriing from
| bringing about any powerful movements of
| the atmosphere. In the tornado the heat
| of the lower air has different origin.
| When a cyclone passes over the surface of
| the country certain peculiar movements Of
the atmosphere which it produces brings
| large volumes OF the warm and moistened
| air to the carth's surface and overlap them
| by cool stratum. The layer of warm,
| moist air tends to rise up for the same rea-
| son that the thin layer of dry air which
| forms the dust-whjri is impellod upward,
| hut on account of its great mass the intcn-
| sity OF the upward ucgence is far greater
In the sand whirl, the upward motion beiIg
! close to the eatthS surface, for the reason
| that the stratum which is irnpeUe1 upward
| is very thin, but in the tornado the stratum
of heated air is usually about $50 feet
| thick; therefore, its whirling action natur
| ally originates at the upper surface of the
| hot layer, for it is at that point the upward
motion begins. Starting in this upper
region, the whirl extends pTogr9s-
sively downward, just as in
! the bath tub the whirl extends progressive
| ly upward from the point at which the mo-
| ton originated, until the whirl may touch
the surface of the earth. When those whirls
! begin they only involve small part of the
air about the point of origin, and so the ac-
| quired velocity of the particles when they
| come to the centre is not great; but grad
| ually they suck air from farther and farther
away. As the field of supply becomes
| larger, and the particles move from greater
| distance, they approach that centre with
| greater and greater speed, and the spiral
| widens and turns with accelerated velocity.
| The longer the journey of the particle, the
swifter it's whirling motion becomes.
Fortunately, the paths of tornadoes are
| ordinarily very narrow-the widest have a
diameter OF less than two miles, the narrow
est of only forty feet. In most cases tor
nado is seriously destructive over width of
not exceeding 500 feet. The length of the
tornado's path across the country does not
commonly exceed thirty miles, and it gen.
orally traverses the distance in about an
hour When the upward corkscrew motion
of the outer part Of the spiral and
the swifter uprush of the air through
the central shaft have drained away
the most of the warm air which gave
birth TO the motion, the tornado dies away.
The equilibrium of the air-inasses Is for a
time restored, the heavier air has fallen
down upon the surface, and the warm air,
spreading literally as it uttains the level to
which it tends, comes into state of quiet.
Assuming the width of the destruction
| brought about by tho storm at 600 feet, and
the length of its journey at thirty miles, we
find that the area of its devastation amounts
TO about 1000 acres, or TO u square area
about two miles on a side. Over this area
the destruction is ordinarily more complete
than that which occurs in the most severe
/ earthquakes